FIG. 1 is a system construction diagram showing one example of a conventional mobile communication system for data communications between an upper station and a base station by a transmission method wherein ATM calls are mapped utilizing an existing leased line.
An upper station 150 is connected to a base station 101 through a network 103 and a leased line 151, and is connected to a leased line interface 152 within the base station 101. Therefore, the upper station 150 and the leased line interface 152 are connected in a 1:1 logical relationship.
FIG. 2 is a typical diagram showing an ATM cell format. An ATM cell 110 is constituted by data of 53 bytes, and is divided into an ATM header 118 in a region of 5 bytes from the head and a payload 117 of the remaining 48 bytes. The ATM header 118 is composed of GFC (generic flow control) 111, VPI (a virtual path identifier) 112, VCI (a virtual channel identifier) 113, PT (a pay load type) 114, CLP (cell loss priority) 115, and HEC (header error control) 116.
VPI 112 is utilized for setting a virtual path between the upper station and the base station. VCI 113 is utilized for identification in the case of the communication of a plurality of data through the set VP (a virtual path).
HEC 116 detects a bit error in the ATM header 118, and is the result of 8-bit CRC coding for 4 bytes of the portion other than HEC 116 in the ATM header 118. The payload 117 is a communication data storage region.
FIG. 3 is a typical diagram showing a leased line (secondary group) frame format which is an example of a leased line frame format.
FIG. 4 is a typical diagram showing the construction of ATM cell termination processing means in the leased line interface 152 in the prior art technique. Data received from the leased line 151 is synchronized in a frame format as shown in FIG. 3 at a leased line frame trailer 120 in the leased line interface 152. After the establishment of the frame synchronization, cell synchronization for establishing the cell boundary position is carried out in an ATM cell synchronization detector 121 to identify the ATM cell 110 which has bean mapped within the frame.
FIG. 5 is a typical diagram illustrating an ATM cell synchronization method in the ATM cell synchronization detector 121 shown in FIG. 4. The ATM cell 110 is fixed in 53 bytes and mapped within the frame. Therefore, from the state wherein cell synchronization has not been established, the ATM cell synchronization detector 121 performs HEC processing 126 of the received signal one bit by one bit, and recognizes a position, at which data free from an HEC error has been detected, as an ATM header 118. Since the ATM header 118 is continuously received in a cycle of 53 bytes, the processing is continued based on the assumption that the next ATM header 118 is present at a position located 48 bytes after HEC-detected position. Stable HEC detection is the point of cell synchronization completion 128.
Upon the cell synchronization completion 128, the ATM cell synchronization detector 121 can extract ATM cells within the frame.
The ATM HEC error detector 122 shown in FIG. 4 performs error detection of HEC 116 in the restored ATM cell 110, and an ATM cell 110, in which an error has been found, is discarded in this functional section because there is a possibility that the error has taken place during data transmission.
In the protocol error detector 124 shown in FIG. 4, upon the receipt of an ATM cell 110 free from the HEC error, confirmation is made on whether or not VPI/VCI is an expected value. When data communication takes place between the upper station 150 and the base station 101, VPI/VCI is set and functions as a parameter at the time of communication with the upper station 150. Therefore, upon the receipt of an ATM cell having an value other than the set value, this ATM cell should be discarded as a protocol error.
FIG. 6 is a typical diagram showing an example of the construction of CCH connection in the prior art technique. In the example shown in FIG. 6, CCH (control channel) is a signal provided for the control of the base station 101, and three CCH signals are provided per base station 101. The upper station uses the CCH signal to control the base station, for example, at the time of connection to a terminal.
In CCH_A 129, VCI 113 is set at “20.” CCH_B 130 has a VCI 113 value of “21,” and CCH_C 131 a VCI 113 value of “22.” Further, DCHs (data channels) also have respective different VCIs 113. All the signals including DCHs have identical VPI 112 “0.” DCHs represent voice data, and are respectively identified by VC.
Since the type of CCH/DCH is identified by VCI 113, the base station 101 should provide the same number of VCI filters 132 as the number of VCIs connected by the leased line 151 to detect an protocol error.
In the VCI filter 132, VPI/VCI does not have a fixed value. Therefore, the value is set at the time of connection to the upper station 150, and comparison is performed for the set value.
FIG. 7 is a typical construction diagram of the protocol error detector 124 shown in FIG. 4. For the ATM cell 110 sent from the ATM HEC error detector 122, VPI/VCI is confirmed by the VPI/VCI detector 134. ATM Cells 110 other than the set VPI/VCI are discarded. CPU 125 receives ATM cells 110, which have not been discarded in the protocol error detector 124, and detects data in the payload 117.
This prior art technique, however has the following problems.
The first problem is that, when a plurality of connections to upper stations 150 have become necessary due to an increase in capacity of the base station 101, the base station 101 should provide the same number of leased line interfaces 152 as the number of upper stations (connected leased lines). FIG. 8 is a typical system construction diagram at the time of the connection of a plurality of upper stations to the base station in a conventional mobile communication system for data communication between upper stations and a base station by a transmission method wherein ATM cells are mapped using an existing leased line.
In signals provided between the upper stations 150 and the base stations 101, like CCH, the number of supports in the base station 101 is fixed. For example, the base station is controlled by an upper station A. On the other hand, data comes from upper station A/upper station B/upper station C. Therefore, the number of CCHs in the base station is limited. In this signal, the upper station connected by the leased line 151, from which the connection of VCI is carried out, is not determined. This is because a redundancy is considered such that, for example, when the upper station A 150a has had trouble, the upper station B 150b as an alternative station connects CCH to prevent breakdown of the system.
FIG. 9 is a typical diagram showing the construction of CCH connection through a plurality of upper stations in the prior art technique. In the conventional protocol error detector 124, filters of VPI/VCI set as shown in FIG. 6 should be provided. Therefore, as shown in FIG. 8, a leased line interface section A 152a, a leased line interface section B 152b, and a leased line interface section C 152c should be provided for respective leased lines.
As shown in FIG. 9, when CCH is connected from each upper station 1 VCI by 1 VCI, however, the VIC filter 132 of CCH not supported by the protocol error detector 124 of the leased line interface sections 152a/152b/152c is not used.
Thus, in the conventional construction, a larger number of VIC filters 132 than necessary should be provided in the protocol error detector 124. This is disadvantageous from the viewpoints of a reduction in size of hardware and a reduction in cost.
The second problem is that, with respect to VPI/VCI set between upper stations, a value falling within a specified range can be independently set for each upper station and, thus, an identical value can be used in each upper station. In FIG. 9, CCH_A 129, CCH_B 130, and CCH_C 131 are different from each other in VCI 113. In fact, however, in all the upper stations, the VCI 113 can be set at “20.”
Thus, since identical VPI/VCI can be used in each upper station, the leased line interface sections 152a/152b/152c cannot identify the upper stations through VPI/VCI.